TLDR: The more times you get things wrong the more it wants to show it to you.
If your first screenshot, it looks like you got every review correct. So you get a steady trend upwards as it promotes to longer and longer gaps between reviews. You know this item so it’s not showing it to you often anymore.
In the 2nd image the most recent 3 reviews were incorrect, so you don’t know that character/word well. The 3rd image is similar - 5 most recent reviews are wrong, wrong, correct, correct, wrong. So 3/5 recently reviews are incorrect. The system is detecting you’re getting those things wrong now, so it’s going to show it to you more often. The ranks on the left are just a measurement of how long before you see this item again. Going from Expert down to Novice only means is that the gap between reviews is getting shorter (Expert is 4 months, Novice II is 4 days), nothing else. This also isn’t bad, this is GOOD! If you’re forgetting something, seeing it more to review it often is correct!
You can read about the exact maths it uses to do the calculations but in general the system is setup so “get stuff right, see it less, get stuff wrong, see it more” which is exactly what you want. That’s the thing with SRS systems - they are NOT exams, they are callendars. In an exam getting a question wrong is bad because you might fail. When using an SRS callendar like HanziHero, telling the system you don’t know something so that it can schedule the next review more accurately is GOOD. It can feel like failure, “oh I got it wrong”, but what you should actually think is “I didn’t remember this, awesome! I now know I need to review this more so I’ll tell HH to schedule it sooner”. Telling HanziHero you don’t know something when you don’t know it is SUCCESS! You didn’t get the question wrong and might fail, instead you correctly scheduled your next review! The only way to “fail” and SRS review is to lie and tell it you know it when you didn’t.
Wow! Thank you! I didn’t know about it especially for the last one (3rd picture). That’s so helpful to learn about SRS.